1. Technical Field of the Invention
This invention most generally relates to methods and apparatus for drying sliced and granular materials and small fruit crops with a heated airflow, and more particularly to distributed airflow containers and airflow circulation systems with simple open loop airflow circuits and complex open and closed loop airflow circuits for batch drying of diced, sliced or granular materials up to berry size to reduce the moisture content.
2. Background
The drying or reduction in moisture content of berries, cherries, grapes, nuts, whole fruits, sliced fruits, meats, and bulk materials provided in the form of granules or slices or small pieces is an old and well informed field of art. For example, the art of harvesting and processing coffee beans from tree-borne xe2x80x9ccherriesxe2x80x9d to the green coffee bean of commerce consists of two principle methods, the xe2x80x9cdryxe2x80x9d method and the xe2x80x9cwetxe2x80x9d method. Either method must result in moisture content equivalent to one third or more of the bean""s weight being removed, to produce a commercial product.
The dry method is the more ancient and rudimentary. The cherries are hand-picked all in one picking, washed, and sun-dried on drying ground or concrete slabs in thin layers, usually for a period of two to three weeks. The beans are heated by solar radiation from above and by secondary radiation from the already warmed concrete slab below, while natural circulation of relatively dry air over the top of the beans slowly leaches out the moisture. The beans ferment during the process, and are turned several times a day to promote even drying. They are covered at night to protect them from reabsorbing moisture during the night time dew point and temperature changes.
In the wet method, only the ripe cherries are picked in any one picking of a tree. It may take three to five sequential pickings in a season over the time it takes between the earliest and the latest cherries to ripen. After the cherries are washed, the outside fruit pulp is removed by machines and the berries are then placed into large concrete tanks to ferment for twelve to twenty-four hours, then poured into concrete sluiceways or washing machines to be thoroughly washed in constantly running water. Then they are dried in much the same way as in the dry method, except that the drying time is shorter. These beans are then processed through hulling machines to remove the remaining layers of skin.
Problems with either method of this art include the inefficient, labor-intensive and lengthy sun-drying time of beans arranged on open air slabs. There have been introduced over the years, other manual, passive solar methods and devices attempting to promote and control air movement in combination with heat, to remove the moisture from bulk crops. Most typically, the beans or other materials being dried, are supported on a foramenous surface or in a container having at least foramenous bottom surface or screen, to permit a greater degree of circulation or air flow in contact with the underside as well as the topside of the bulk materials.
Various electrical powered and/or fuel-fired dryer systems have also been used to try to accelerate the drying time and prevent mold problems. There are many patents that describe related technologies and devices. Most of these alternatives add expense and complexity to an otherwise simple process. Failing to safeguard the beans from excess moisture, in particular the formation of mold during the drying process is crucial as the value of the crop drops dramatically if mold occurs. Over drying can also occur using accelerated methods; this also affects the quality and value of the crop. A sampling of the art of convective and low pressure air drying systems is included to provide context for the reader:
Stokes"" U.S. Pat. No. 4,490,926 (1985) discloses a solar drying device and method for lumber, tobacco and grain. It includes a solar collector, a drying chamber, and a dehumidification system. The background section mentions solar heated kilns and dryers with easy access and containerized methods, wheeled vehicles or carts, for moving materials into and out of the dryer. Insulation and double glazing of light-admitting sheet materials is discussed, as is passing air between a drying chamber and a dehumidifying chamber. The focus is on drying and reusing the air.
Sutherland""s U.S. Pat. No. 5,584,127 (1996) is a recent patent for a solar powered fruit dryer. The focus of the apparatus design is on recirculation of a portion of the drying gas. It refers to air circulating through perforated shelves (col. 4, line 32) upon which the materials are arranged. Column 4, line 60, describes the physical embodiment in some detail, including air flow volumes.
Andrassy""s U.S. Pat. No. 5,001,846 (1991) is a solar drying apparatus with a translucent sloping top and means for evacuating the condensation from the moist air. The specification describes a perforated or porous tray on which the materials are arranged for drying. A solar powered fan forces drying air vertically through the porous tray.
Mullin""s U.S. Pat. No. 4,099,338 (1978) shows an elaborate, solar-assisted dryer for tobacco, onions, titanium dioxide drying, polyester fiber setting, and roasting nuts and cereals. The focus appears to be on ratios of solar heated makeup air in the circulation system to save fuel. The material is dried on a foraminous conveyor belt.
O""Hare""s U.S. Pat. No. 4,501,074 (1985) is a convection powered solar food dryer that discloses a solar collector on the inlet side for heating intake air, and a vertical solar tower or column to accelerate the convection of warm air through the system by suction. The actual drying chamber can be removed from the solar devices at each end of the convection system. The materials are arranged on shelves in the drying chamber.
Steffen""s U.S. Pat. No. 4,045,880 (1977) is a solar grain drying apparatus. It discloses a fan forced down draft eave inlet solar roof heating system, that then drives the drying air up through the perforated floor of the central drying chamber. The air is then exhausted upwards roof exhaust fans in the drying chamber ceiling.
Muller""s U.S. Pat. No. 1,556,865 (1923) is a solar powered dryer system for vegetable matter, consisting of a series of circumferential racks with inlet perforations in the sidewalls and internal shelf brackets in the corners for holding drying shelves or trays. The racks are configured for interlocked stacking underneath a solar collector roof which has a central exhaust vent.
Pietraschke""s U.S. Pat. No. 4,391,046 (1983) is a solar heated grain drying system featuring an inlet manifold receiving multiple collector pipes and a fan blowing the intake air up through a perforated floor in the drying chamber.
Sweeny""s U.S. Pat. No. 278,199 (1883) is a coffee roaster showing perforated drums for containing the coffee beans, configured to revolve within a heated chamber. The drums are feed by hoppers through the ends. The drums use internal vanes to distribute the beans or other materials lengthwise, particularly for loading and unloading the drums. Heating is by other than solar means.
Danford""s U.S. Pat. No. 4,263,721 (1981) is a tobacco curing and drying structure that is configured for adding makeup air, using a heat exchanger and means for partial recirculation.
The drying of coffee beans is exemplary of the prior art. The drying or dehydrating of fruits, nuts, vegetables and other food crops and naturally granular or crushed or sliced materials is a much frequented subject in the prior art. It is noteworthy that a sliced and dried piece of fruit has a significantly higher value than the freshly harvested product. While coffee and related bulk crops were the subject of the parent applications, the principles disclosed there are extended in both content and application in the disclosure that follows.
The invention in it""s simplest form is a low pressure airflow dryer or dehydrator system for reducing the moisture content in berries, cherries, grapes, nuts, whole fruits, sliced fruits and garden foods, sliced meats, and bulk materials in the form of small pieces or granules or slices, and sliced crops in particular. Materials for which the invention is suitable can be divided into three categories by size and shape. The first category encompasses granular or crushed crops and whole nuts and berries; such as rice, whole coffee beans, cocoa beans, vanilla beans, crushed coconut, blueberries, strawberries, cranberries, and other seeds, pods, grains and materials having naturally occurring small specimens, or being easily reduced to small pieces by crushing, chipping, cutting, freezing and breaking, or other mechanical means, of a nominal average diameter between one quarter and about one inch, and having sufficient structural integrity to be disposed at least several inches deep, preferably as deep as three or four feet, without damage that would affect its dried value. In the case of relatively hard granular bulk materials such as coffee beans, commercial embodiments may utilize much greater depths in combination with complex airflow circuits and automated loading and unloading mechanisms.
The second category is bulk materials including fruits, vegetables and other crops, specimens of which can be easily reduced to slices of uniform thickness between about one quarter and one inch, and still have sufficient structural integrity to be stacked edgewise at least several inches and preferably to as high as three to four feet within the apparatus of the invention for drying, without damage that would affect its dried value. Examples include fruits such as apples, pears, mango, papaya, and carrots.
The third category is bulk materials including crops, specimens of which can be easily reduced to slices of uniform thickness as described above, but which may not have sufficient structural integrity to be stacked vertically on edge, and so are preferably handled in a horizontal plane without stacking. Examples include crops such as tomatoes, peaches, watermelon and bananas.
At the core of the system, there is a specialized bulk crop container specially configured to form a system of open wall airway channels uniformly distributed throughout the container and hence the selected bulk material when it is added to the container, the airways connecting through openings in the top and bottom or through opposing sides of the container as airflow inlets and outlets connecting to a closed or semi-enclosed primary airflow circulation system so that a distributed airflow can be directed through the airway network of the container to leach excess moisture efficiently from the bulk material. The container may be integral to the dryer system or removable.
The primary airflow system has a heater to elevate the air temperature so as to be able to absorb more moisture. The heater may be a heat exchanger of any type or a heat generator such as an electric heating element. The airflow is maintained by an air mover of any type, most typically a simple fan. The flow rates and pressure drop across the container are not excessive, generally within the range of standard HVAC (heating, ventilation and air conditioning) industry practices.
A more limited secondary or exterior airflow or circulation path provides a partial exhaust and makeup air supply to the primary airflow system, so that moisture levels are kept below the saturation level. A heat exchanger using the inlet and exhaust airflows of the secondary airflow system may be employed to elevate the temperature of the inlet or makeup air so as to hold more moisture, by scavenging heat from the exhaust airflow.
The key to creating an open-wall airway network distributed throughout the container is the use of an internal structural network of minimal volume that provides an array of open face grooves or channels spanning the height and width of the container. The width of the open face each groove or channel is specified to be sufficiently narrow to prevent more than partial penetration into the groove by an average size granular type material being dried, and still pass a useful volume and rate of drying air without undue restriction or pressure drop. The parallel set of dividing partitions between the airflow channels provides an adequate surface area and sufficiently closely spaced support grid to support the sliced materials being dried. The depth of the groove or channel is sufficient to assure an airflow passageway will remain open the full length of the groove or channel, when the container is full of the bulk or sliced material.
An efficient form of the required internal structure of the container is a series of parallel partitions or airflow plates, dividing the container into a parallel set of uniformly thin bays or compartments, preferably in the order of three eights to one inch in width. The bays may be arranged in the vertical plane or the horizontal plane. In either case the opposing faces of each bay feature a parallel set of grooves running the full height or width of the partition, and terminating at or actually projecting through a foramenous end wall or bottom panel such that the airway formed by the groove is accessible to an airflow that is ducted or channeled to that wall or bottom panel. It will be apparent that the partitions themselves consume width in the container between compartments, in order to provide the unobstructed, uniformly distributed air channels that are a hall mark of the invention.
As described, each groove or channel has an open face exposed to the bulk material, while being sufficiently narrow to prevent the pieces or slices of materials from penetrating into the groove. This provides a significant surface area of the material with direct or near direct exposure to the drying effects of the airflow in the groove or channel. Closely adjacent airflow channels or grooves on each airflow plate, and closely spaced airflow plates uniformly distributed within the container volume, assure a uniform and relatively quick penetration of the drying effects of the airflow as to the material in the container.
Practical embodiments of partition material, as will be discussed more fully below, include ribbed panels, where both sides of a panel are configured with parallel sets of raised ribs, the spaces in between which are grooves; and corrugated panels, where both sides of the panel present to their respective bays or compartments, a parallel array of ridges and grooves. Raised ribs or round corrugations, rather than sectional or box corrugations with flats, have a further benefit of offering only a tangential point of contact to the materials being contained. Other forms and embodiments of the internal structure are within the scope of the invention.
The preparation of fruit or other materials needing to be sliced for loading and drying requires the fruit to be sliced into uniformly thick slices that will fit closely within the width of the drying compartments and slide into a closely packed arrangement without binding. There is no particular orientation required of the fruit for slicing, so the slicing can be easily automated or semi-automated for speedy slicing. In the case of category two vertical orientation of the bays and vertical stacking for drying, the edgewise oriented column of slices in the compartments must not be so tall as to seriously crush the slices at the bottom. However, this has not been a problem with containers suitably sized for manual handling and compartments in the order of three eighths or one half inches wide and up to 30 inches tall. Containers for category three sliced materials are arranged with partitions in the horizontal plane, so that each partition acts as a ribbed tray for the bay above it, suspending the slices sufficiently on the air channel partitions to permit drying airflow beneath the slices as well as over them.
The cycle of loading and unloading of the bulk materials into and out of the dryer system may be enhanced by configuring the container or containers with bottom panel gates or sliding gates which can be opened to dump the contents of each bay, and closed for refill and operation of the dryer, without removing the container from the system. An optional vibrator may be attached to the container or framework of the apparatus to aid in filling and emptying the container. The vertical bay container may be manually filled with sliced materials through the open top, more akin to how granular bulk crops such as coffee beans are loaded, with greater speed and efficiency by carefully metering the sliced materials out of a dispensing container so as to flow the slices into the open end of the container with an orientation parallel to the partitions.
An alternative method for loading of materials, and in particular category three materials, is to arrange the container so that the airflow plates are in the horizontal plane. The grooved plates or panels can then be removed sequentially or collectively through an open end of the container for manual or automated placement of a single layer of slices on each panel and reinsertion into the receiving slots of the container. A loading rack may be used to receive and deliver the full set of plates to and from the container. The loading and unloading may be further automated for higher volume commercial practices.
During the drying process, there may be some tendency for some types of fruit or other materials to stick lightly to the panels. When the drying cycle is complete, a light sweep over the panel surface releases any stuck slices. If desired, an antistick coating may be applied to the plates prior to use, or the plates may be fabricated with a non-stick surface.
As is apparent from the above description, by arrangement of the airflow panels or partitions, or by reorientation of the container, the airflow through the container with vertical bays can be arranged to be vertical or horizontal; whereas in a container configured with horizontal bays, the airflow is constrained to horizontal although it may be in either of the two orthogonal horizontal axis. Upwardly vertical airflow is particularly useful for very low airflow pressure systems such as passive solar systems where thermally generated convective airflow with minimal head pressure can be applied to a single level container. Not withstanding, short horizontal airflows may also used.
Alternatively, a user may, by using a forced airflow system, provide a much greater pressure and volume of air through the container than typical passive solar systems. This makes the larger container systems with horizontal airflow plates useful, whether configured with vertical or horizontal drying bays. Forced air circulation with or without a supplemental heat source for adding more heat to the air, speeds up the process. Using a heat pump as the air mover and dryer adds significant efficiency to the process with its ability to cycle air temperature so as to squeeze out the moisture and then reheat and recirculate the air. The user may obtain either faster drying time of a small batch of materials by pushing more air through the dryer, up to a maximum useful rate of extraction of moisture; or greater batch capacity by using larger and more complex containers with either vertical or horizontal airflow networks, interconnected with ductwork to link the containers.
The container is scalable and adaptable to smaller and larger dryer systems utilizing heat exchangers, solar radiation or other power sources for generating a warm, relatively dry, low to moderate pressure airflow. The container, inserted or connected to the airflow plenum of the system for both inlet and exhaust, absorbs the full flow of drying air through its interior. The internal construction provides a baffle effect on the pressure side of the container, which promotes very uniform distribution of airflow through the materials and even drying, overcoming a significant disadvantage of other systems.
In the passive solar drying of bulk crops such as coffee and grains, nuts and berries, and sliced fruits and vegetables, airflow is generally more limited than heat, due to the relatively low differential pressure that can be generated in low cost, practical, solar radiation dryers. It takes many hours or days to affect a significant reduction in moisture levels in the passive solar drying of crops. The relative amount of airflow to which the crops are directly exposed has been demonstrated in passive solar dryers to be the more significant factor to the dryer""s utility and efficiency, than simply adding heat. Too much heat with too little air will do more damage than good. It is therefore important to configure solar powered dryers to obtain maximum flow from a relatively dry air source, and maximum exposure of the materials to the dry air flow, while retaining a low cost structure and a simple bulk container handling system.
The principle functional components of a primary dryer system of the invention are a warm, dry airflow generator, a bulk materials container configured to provide the uniformly distributed open channel airways network of the invention, structure for supporting the container within the primary dryer system in such a way as to constrain air flow to flowing through the airways of the container, and features of the container by which it can be filled and emptied.
As introduced above, a further aspect of the invention provides a complex airflow system with a closed primary loop and an open secondary loop. As distinguished from the simpler case above, there is a closing of the circulation loop or path of the primary airflow system through the container as a recirculating airflow with a good airflow rate and pressure drop through the airflow plates of the drying container and hence through a larger, lower airflow rate, mixing plenum, the pressure and airflow rate being maintained by airmover mechanisms. The airflow is heated by any suitable means and the overall enclosure may be insulated to conserve heat. Added to that is a secondary, open airflow circulation loop of more limited airflow rate that provides for partial exhaust and makeup air to the primary closed loop airflow path for removing excess moisture. This permits configuring the total system to optimize each functional element of the process at the lowest total energy cost, as will be further described below.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein I have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by me on carrying out my invention.